A jet aircraft is often provided with an on-board auxiliary power unit (APU), a small gas turbine engine one function of which is to provide starting power for the main engine or engines. The auxiliary power unit is also used to operate various aircraft components via the hydraulic and electrical systems while the aircraft is grounded, usually for testing purposes. When in the testing mode, the auxiliary power unit must be mechanically disconnected from the engine or engines.
Some aircraft employ air turbine starters (ATS) operated by on-board compressors, but in that case the compressor motor is often used in the same way as an auxiliary power unit to operate the hydraulic and electrical systems for testing purposes. It too must be disconnected from the engine.
A critical aspect of the design of many military aircraft relates to the time required to change from the test mode to the fully operational mode in which the auxiliary power unit can be used to start the engines. If the re-connect time is too long, the aircraft must be considered non-operational and unable to respond to an attack. In some geographic areas, the re-connect time must be measured in seconds.
The solution to this reconnect problem relied upon to date has been the use of a torque converter associated with each engine. In a two engine configuration, the auxiliary power unit drives the first engine through the corresponding torque converter until it is started and then the first engine drives the second through a cross-drive and its torque converter until it starts. There have, however, been significant problems associated with the use of torque converters for this purpose, and a superior engine starting apparatus and method has been sought.
Some important problems associated with the use of torque converters relate to the high sensitivity of the converters to temperature changes. When the temperature is low, for example, the converter fluid becomes excessively viscous and the resulting high torque can cause shafts and other components to fail. Moreover, the viscous fluid sometimes will not flow through small converter passages important to the operation and control of the device. Other disadvantages to the use of torque converters relate to the fact that they do not provide a linear relationship between speed and torque. In addition there are problems arising from the foaming action of converters when operated in the cavitation mode, as is necessary for cross-drive purposes.
Despite the known disadvantages of torque converters, no satisfactory alternative starting power drive line has been devised. Torque converters alone have had the required capability to mate the engine to the torque/speed curve of the auxiliary power unit over the entire speed range that takes the engine from a rest condition to a point at which it is not only self-sustaining but it is able to accelerate quickly to full power. The torque that must be transmitted to the engine drive shaft is of an order of magnitude of about 150 ft. lbs.
Applicant has considered the replacement of each torque converter by a transmission having a plurality of parallel gear trains and a series of clutches that allow these gear trains to be employed successively. However, the number of separate gear trains required to cover the operating range of the starting apparatus would appear to be excessive, increasing in size, weight and complexity beyond acceptable limits. Moreover, it would prove very difficult if not impossible to shift smoothly between successive gear trains, particularly during the resist phase of the starting procedure.
To reduce the number of separate gear trains would require greater slippage in the clutches. Conventional clutches, either wet or dry, that would be capable of handling the torque loads imposed cannot withstand the necessary slippage and the high energy dissipation requirements. Typically these clutches would have to be fully engaged within less than about one second to avoid burning out.
A primary objective of the present invention is to provide a relatively simple but highly effective starting power drive line for jet aircraft and method of operating the same that overcomes disadvantages of previously known apparatus that employ torque converters. A further objective is to avoid the need for a transmission having a large number of successively available gear ratios and to provide clutches that are capable of handling the high loads and energy dissipation requirements.